Image dissector with field mesh near photocathode



Dec. 27, 1966 R, H. CLAYTON IMAGE DISSECTOR WITH FELD MESH NEAR PHOTOCATHODE Original Filed March 6, 1963 WNU INVENTOR.

ROBERT h'. CLAYTON ATTQRNEY United States Patent O 3,295,010 IMAGE DISSECTOR WITH FIELD MESH NEAR PHOTOCATHODE Robert H. Clayton, Fort Wayne, Ind., assignor to Interp national Telephone and Telegraph Corporation, Nutley,

NJ., a corporation of Maryland Continuation of application Ser. No. 263,321, Mar. 6, 1963. This application May 25, 1966, Ser. No. 552,877 8 Claims. (Cl. 315-11) This invention relates to image tubes and particularly to a novel image dissector tube having improved electron optics. The present application is a continuation of previous application Serial No. 263,321, filed March 6, 1963, and now abandoned.

Image dissector tubes have been used to provide electrical output signals in response to an impressed optical image. A photocathode surface at one end of the tube emits electrons in accordance with the elemental light impinging thereon. The electrons are accelerated through a series of electrode rings having stepped potentials applied thereto and are at the same time electromagnetically focused and deflected or scanned across a minute aperture in the focal plane of the image. Output currents are transmitted through the opening corresponding to the successive elemental areas'of the image sweeping over the analyzing aperture, with an electron multiplier structure providing an amplified signal at the output end of the tube. The output may then be employed, for example, in television or star-tracking systems. Examples of prior art tubes of this type are found in U.S. Patents No. 2,459,778 issued l an. 18, 1949 and No. 2,913,610, issued November 17, 1959, and are assigned to the same assignee as the instant application.

Attempts to miniaturize and improve this device have been limited due to the use of a plurality of accelerating electrodes and an external voltage divider network applying the various stepped potentials thereto. The electrodes require a large number of leads through the tube envelope and introduce field disturbances. In addition, electrons in transit from the cathode to the aperture plane may collide with gas molecules -to generate ions which are directed toward and cause deterioration of the emissive surface. One proposed solution has suggested the use of a resistive spiral coating on the tube wall. However, this is difficult to fabricate and permits spurious charges to accumulate on the wall between spirals.

It is therefore the primary object of the present invention to provide a novel configuration for an image dissector tube which permits simplification of the structure with increased resolution and longer life.

It is another object to eliminate the use of a plurality of accelerating electrode rings and reduce ion bombardment and damage to the photocathode.

These results are accomplished by use of a novel field mesh or screen in close proximity to the photocathode and supported on or adjacent a unipotential hollow cylindrical or tubular electrode. Acceleration occurs between the cathode and mesh, with deflection taking place in an electrostatic field free space within the cylinder, thus minimizing defocusing and reducing the number of electrodes and leads. In addition, ions generated in this space are less subject to acceleration by electrostatic fields which may direct the ions toward the photocathode. The details of the invention and other objects and advantages will become apparent from the following description and accompanying drawing wherein:

FIG. 1 shows a cross section of the novel -tube structure, and

FIG. 2 shows a variation employing aspaced apertured plate.

An optical image represented by arrow 10 is impressed ICC upon a photoemissive layer 12 on the inner surface of faceplate 14 of a glass envelope 16. Electrons are emitted from the photocathode to form an electron image in accordance with the light pattern and are accelerated toward a field mesh or screen 18 positioned parallel and adjacent the emissive surface. The cathode, for example, may be placed at a reference potential and the screen at approximately +300 volts, with a spacing of from 0.125 to 0.500 inch therebetween, such that no focusing of the mesh will occur, as seen at the aperture. The screen encloses the open end of a hollow cylindrical anode `20 which may directly support and connect to the screen at the same potential or may be spaced therefrom and at a small negative potential with respect thereto. For example, the spacing between screen and anode may be in the order of 0.002 to 0.125 inch and the anode potential may be about +295 volts. This difference in potential aids in retarding the ions from striking the cathode and thus prevents deterioration of the emissive surface.

The cylinder is made of a non-magnetic material such as Nichrome and forms a unipotential wall electrode having a field free region in which the electron image is magnetically focused and deflected without disturbance from accelerating electrostatic fields. A known form of focusing solenoid 22 surrounding the tube envelope causes the electron image to be focused electromagnetically in an integral number of focusing loops in the plane of an apertured plate 24 at the opposite end of anode 20. The deflection coils 26, similarly positioned concentrically around the tube envelope within the focusing coils, produce horizontal and vertical electromagnetic scanning of the image across the aperture in a well known manner. The aperture thus analyzes the image to provide output currents representing successively scanned elemental areas. The electrons are thus signal modulated and then directed through an adjacent series of secondary emissive electron multiplying dynodes 28 having suitably stepped positive voltages applied thereto to provide an amplified output signal at a collector electrode 30 which is connected to a utilization device, such as a television transmission or star-tracking system.

As shown in FIG. 2, the apertured plate 24 may enclose the end of the anode 20 or be positioned in close proximity and at a slight negative potential with respect thereto. Such separation may provide compensation for non-uniformities in focusing and aid in preventing back scattering secondary electrons from entering the multiplier. In addition positive ions from the drift tube may be attracted away from the photocathode to further prevent damage to the emissive surface.

Since total acceleration occurs in the first fractional inch before the screen, deflection is accomplished in a field free region so that defocusing and other disturbances are minimized. Thus an improved resolution may be obtained. Alternatively, the original resolution may be maintained with use of lower operating voltages or a smaller photocathode. In addition, elimination Iof the complex accelera-ting structure permits miniaturization of the entire tube. A typical tube, for example, may thus have a total length of approximately 8 inches and an outer diameter of about 21A inches, including the coils, a photosurface diameter of 1.1 inches, and an aperture diameter of 0.001 inch. With this type of configuration, resolutions of over 2000 lines on the photocathode. diameter are obtainable.

It may thus be seen that the present invention provides a novel miniature image dissector tube having a more efcient structure with improved optical characteristics. While several embodiments have been illustrated, it is apparent that the invention is not limited to the exact forms or uses shown and that many other variations may be made in the particular design and configuration without departing from the scope of the invention as set forth in the appended claims.

What is claimed is:

1. An image dissector `tube comprising:

an envelope;

a photoemissive cathode layer on lthe inner surface of one end of said envelope, said layer emitting electrons in response to an applied optical image to form a corresponding electron image;

a longitudinal tubular anode having an open end adjacent said cathode surface;

a mesh screen accelerating electrode disposed at said open end and positioned in close proximity and parallel to said cathode surface;

means applying an electron accelerating lield potential to said screen and anode with respect to said cathode;

means providing a scanning aperture at the opposite end of said anode;

first electromagnetic means surrounding said tube and anode for focusing said electron image in the plane of said aperture;

second electromagnetic means surrounding said tube and anode for deecting said electron image across said aperture, said aperture permitting passage of electrons therethrough in accordance with successively scanned elemental areas of said electron image; electron multiplier means positioned adjacent said aperture to provide an amplified signal in accordance with said scanned image; and means for deriving an output signal from said multiplier means.

2. The device of claim 1 wherein said screen is connected to said anode and at the same potential as said anode.

3. The device of claim 1 wherein said screen is spaced from said anode and at a small positive potential with respect to said anode to retard ion bombardment of said cathode surface.

4. The device of claim 1 wherein said tubular anode is formed of non-magnetic material.

5. The device of claim 1 wherein said screen is spaced from said cathode 0.125 to 0.500 inch such that focusing of the screen at the aperture is prevented.

6. The device of claim 1 wherein said means providing a scanning aperture is a plate positioned in close spaced relation to the end of said anode.

7. The device of claim 1 wherein said means providing a scanning aperture is a plate enclosing the end of said anode.

8. The device of claim 6 wherein said plate is at a small negative potential with respect Ito said anode.

References Cited by the Examiner UNITED STATES PATENTS 2,412,086 12/1946 Hallmark 178-7.2 2,459,778 l/l949 Larson 178-7.2 2,541,374 2/1951 Morton 178-7.2 3,047,758 7/1962 Rome 315-31 X DAVID G. REDINBAUGH, Primary Examiner. T. A. GALLAGHER, Assistant Examiner. 

1. AN IMAGE DISSECTOR TUBE COMPRISING: AN ENVELOPE; A PHOTOEMISSIVE CATHODE LAYER ON THE INNER SURFACE OF ONE END OF SAID ENVELOPE, SAID LAYER EMITTING ELECTRONS IN RESPONSE TO AN APPLIED OPTICAL IMAGE TO FORM A CORRESPONDING ELECTRON IMAGE; A LONGITUDINAL TUBULAR ANODE HAVING AN OPEN END ADJACENT SAID CATHODE SURFACE; A MESH SCREEN ACCELERATING ELECTRODE DISPOSED AT SAID OPEN END AND POSITIONED IN CLOSE PROXIMITY AND PARALLEL TO SAID CATHODE SURFACE; MEANS APPLYING AN ELECTRON ACCELERATING FIELD POTENTIAL TO SAID SCREEN AND ANODE WITH RESPECT TO SAID CATHODE; MEANS PROVIDING A SCANNING APERTURE AT THE OPPOSITE END OF SAID ANODE; FIRST ELECTROMAGNETIC MEANS SURROUNDING SAID TUBE AND ANODE FOR FOCUSING SAID ELECTRON IMAGE IN THE PLANE OF SAID APERTURE; SECOND ELECTROMAGNETIC MEANS SURROUNDING SAID TUBE AND ANODE FOR DEFLECTING SAID ELECTRON IMAGE ACROSS SAID APERTURE, SAID APERTURE PERMITTING PASSAGE OF ELECTRONS THERETHROUGH IN ACCORDANCE WITH SUCCESSIVELY SCANNED ELEMENTAL AREAS OF SAID ELECTRON IMAGE; ELECTRON MULTIPLIER MEANS POSITIONED ADJACENT SAID APERTURE TO PROVIDE AN AMPLIFIED SIGNAL IN ACCORDANCE WITH SAID SCANNED IMAGE; AND MEANS FOR DERIVING AN OUTPUT SIGNAL FROM SAID MULTIPLIER MEANS. 